How can I scale particle size over lifetime within a keyed physics system
Currently it is not really possible to drive particle size with a driver. One can drive them so they show in the 3D View with different sizes, but the render engines (BI and Cycles) don't see the size changes, unfortunately.

See the report in the Blender TODO tracker here: http://projects.blender.org/tracker/?func=detail&amp;atid=264&amp;aid=34946&amp;group_id=9 . Some examples are included.